On this day in 1970, U.S. virologist David Baltimore published a breakthrough paper in Nature describing reverse transcription. The process enables some viruses to insert their genetic material into the DNA of healthy human cells, which can lead to tumors and other diseases.

Baltimore and oncologist Howard Temin independently discovered the enzyme, later called reverse transcriptase, that enables so-called retroviruses—including HIV—to use RNA as a template for making DNA. Their work showed that genetic information not only flows from DNA to RNA (as it does in normal genetic transcription), but can also move from RNA to DNA. The finding opened new avenues for the study of cancer and for genetic engineering. Baltimore, Temin, and virologist Renato Dulbecco shared the 1975 Nobel Prize in physiology or medicine for the breakthrough.